A secret network of chambers of "cathedral-like" proportions has been discovered in the base of Isambard Kingdom Brunel's most celebrated bridge.
Civil engineers had believed that the 110ft-high support on one side of the Clifton Suspension Bridge in Bristol was made of solid stone. But a dozen vaulted chambers up to 35ft (11m) deep were discovered by engineers during tests on the 160-year-old structure.
Although hollows were a common feature in bridges and railway stations in Victorian times because the design saved on raw materials, no plans of the base of the Clifton bridge survived and it was always assumed the support was solid.
Brunel, an acclaimed engineer and inventor, designed the bridge after winning a competition but died before its completion in 1864, when it became the longest single-span road bridge in the world.
He was the chief engineer of the Great Western Railway and his work includes the Royal Albert Bridge over the River Tamar in Saltash, near Plymouth in Cornwall.
In the Clifton bridge, the chambers of red sandstone have been built over two tiers and can be accessed only by abseiling through a very narrow shaft about 3ft (60cm) wide.
They are empty except for a number of stalagmites, some of which are 10ft (3m) long. The Bridge Master, John Mitchell, said: "It was always assumed that it was solid and the theory was reinforced in 1969. A borehole investigation at the time happened to go through a solid part of the structure."
Earlier this year, electronic surveys indicated the possibility of a shaft and, following excavations, abseiling specialists discovered the tunnels leading to the chambers.
Seven of the chambers are on an upper level and five more are on a lower level, built directly on to the rock of the Avon Gorge, with linking tunnels and shafts.
There were no signs of life in the hollows, which had not been entered since the abutment was completed in 1840.
Mike Rowland, manager of the bridge's visitor centre, said: "We were amazed at their scale. People who have been there have described it as amazing. It has cathedral-like proportions and can be a little eerie."
David Dawson, a trustee of the bridge and a railway historian, said hollow structures supported several Victorian-built stations in London and were a common feature of rural stations.
He said: "This discovery is a surprise, especially given the scale of the abutment.
"But it was not unusual at that time to use this method because stone and bricks were expensive relative to labour."
According to Mr Dawson, Brunel was one of the first civil engineers to exploit the benefits of "voided construction" in reducing the cost and the weight of a structure.
He incorporated the method into the railway stations he built at Paddington and Bristol Temple Meads, both of which are supported by vaulted arches built underground.
The chambers at Clifton Suspension Bridge are closed to the public but an exhibition detailing them has gone on display at the visitor centre.
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